Preparation

Combine the cumin seeds, mustard seeds, coriander, turmeric, cayenne, fennel seeds, and ginger in a small bowl. Stir well to blend.In a large sauté pan set over medium heat, heat the 1/4 cup oil. Add the onions and cook, stirring occasionally, until they are caramelized to a golden brown, 10 to 15 minutes. Reduce the heat to low and add the spice mixture. Cook for 1 to 2 minutes to toast the spices. Transfer the mixture to a bowl. Add the yams, flour, egg, and salt, and mix well to combine. Form the mixture into cakes that are about 3 inches in diameter and 1/4 inch thick, and put them on a baking sheet.Set a skillet over medium-low heat and add the remaining 1 tablespoon oil. Working in batches, sauté the cakes for 3 to 4 minutes on each side. They are done when they are a crispy golden brown and the sweet potato is cooked through.Serve warm, topped with the yogurt and scallions.* Black mustard seeds: Small round seeds of the black mustard plant, these are much more pungent than white mustard seeds and used extensively in Indian and other Asian cooking.